Single-plasticity cases now can only use the "safe" deactivation.
That means they never try to switch deactivation schemes and waste
time in copying arrays and so on.

Dumb deactivation/reactivation now only occurs if the stepsize
is less than max_stepsize_for_dumb.  This is to prevent "dumb"
consuming resources on situations where a stepsize cut is needed.

Fixes #4073
Merge pull request #4074 from WilkAndy/dumb_4073

Single-plasticity cases now can only use the "safe" deactivation.